My boyfriend is being unbearably insecure about my ex's penis size?

You could tell your b/f to move on but will he? Insecurity is akin to jealousy really--especially as he doesn't even like your ex b/f. This is NOT going to go away unless you lay down some ground rules which he either accepts or does not accept. At the end of the day you do not need this cr4p to deal with--either he deals with it or he ships out--simple--you don't need this as i said. (P.S--he wouldn't want to know about MY dimensions then!)